A need for development of next-generation clean energy has increased with the recent environmental contamination and the exhaustion of fossil fuel. A solar cell is a device that directly converts solar energy into electric energy. Since the solar cell hardly causes the air pollution, has infinite resources, and has a semi-permanent lifetime, the solar cell is expected to be an energy source that can solve the future energy problem.
The CIS or CIGS thin film is one of group I-III-IV compound semiconductors and has the highest conversion efficiency (about 20%) out of thin-film solar cells prepared experimentally. Since the CIS or CIGS thin film can be fabricated with a thickness of 10 or less micrometers and has a stable characteristic even in a long-term use, it can be expected to be a low-cost and high-efficiency solar cell which can be replaced for silicon.
Particularly, the CIS thin film is a direct-transition semiconductor and can be formed with a small thickness, the band gap is 1.04 eV which is relatively suitable for photoelectric conversion, and the light-absorbing coefficient thereof has a relatively large value out of those of known solar cell materials. The CIGS thin film is a material obtained by replacing a part of In with Ga or replacing Se with S to improve the low open voltage of the CIS thin film.
However, since the CIS or CIGS thin film employs In or Ga element which is expensive, it has problems in that the preparation cost thereof is relatively high and the band gap thereof is relatively low.
To improve the efficiency of a solar cell and to reduce the cost thereof, there is a need for a new material that can enhance the band gap and contains a low-cost element and a preparation method thereof.
Recently, to develop a new material in which In element can be replaced with a low-cost element, a CZTS-based thin film in which In is replaced with Zn or Sn has been vigorously studied. However, the CZTS-based thin film has a problem that its efficiency is lower than that of the CIS or CIGS thin film.
The inventors have studied about a compound semiconductor thin film that can be used as a light-absorbing layer of a solar cell and that has a high conversion efficiency and a reduced preparation cost and developed a Cu—In—Zn—Sn—(Se,S)-based thin film for a solar cell that can reduce the amount of In to be used and can exhibit a conversion efficiency similar to that of the known CIS or CIGS thin film and a preparation method thereof, thereby completing the invention.